Maybe it's to remind us that life isn't always black and white...
I had assumed it was my tinker toy of a puter until other folks said they saw it too.
Maybe it's a secret two way mirror type thing planted to observe us dropping burrito bits into our keyboards ( or maybe I am the only oe who does that)
I'm kind of getting used to it and hadn't really noticed it today until you pointed it out.

MAYBE... it's an alien plot to take over by slowly eating away our screen, starting with butch-femmer's.
It would have worked too if you hadn't said something!